I don't really know much about directx, etc, but I THINK this can't be done, without starting the game from 0.
Normally (as far as I know), the directX-version is chosen by choosing the game engine, which is, in case of WoW, pretty old.
And if there is a chance to get DirectX11 you have to bring in the information, DX11 needs (better graphics don't only come from the rendering system).
Well, I know nothing :D but that's what I think, it would be like.